It has been proposed to dry form a web of cellulosic material and to simultaneously corrugate and consolidate it in Cdn. Pat. No. 955,094 issued Sept. 24, 1974--Bodycomb. It has further been proposed to produce a linear corrugated web, i.e. a web having a corrugation extending in the longitudinal direction by simultaneously corrugating and consolidating a dry formed web (see Cdn. Pat. No. 955,095 issued Sept. 24, 1974 to Flewwelling).
The concept of forming a corrugated board with the corrugations extending longitudinally thereof i.e. the longitudinal or machine direction of the liners has been a goal of the corrugated paper board industry for many years but it was not until the Bodycomb invention referred to hereinabove was modified as taught by Flewwelling that it was possible to do so in any reasonable width. This is because linearly corrugating a web of paper requires condensing the web widthwise as it is corrugated longitudinally thereby making a corrugating and handling operation extremely difficult and limiting the maximum width of the corrugated web significantly.
The corrugating technique of Flewwelling eliminated these problems of condensing a web laterally. However, the technique created other problems. When a wide web, say, 96", is consolidated, the press nip for consolidating the web must be at least 96" long and it is impossible to provide a suitable press operable at the required pressures and temperatures without resorting to extremely complex construction and very accurate control of temperatures of the mating rolls. The problem is further compounded because with a linear corrugated web the width of the web determines the maximum circumference of a corrugated box and in many cases this circumference is quite large.
It will be noted that with mating ridges and grooves extending circumferentially of the rolls as required for linear corrugating, any relative deflection of the rolls changes the angular relationship of these ridges and grooves and prevents their accurate mating e.g. if the rolls bend along their longitudinal axes by deflection under pressure in the nip the central planes (radial) of the ridges on each roll will tend to converge toward the centre of the nip whereby the ridges at the ends of one roll will not align with the mating grooves on the other roll.
Consolidation to form a linear corrugated web further complicates the operation since the inter-meshing of the two rolls wherein minor differences in thermal expansion between the mating press rolls forming the nip (the consolidation is at elevated temperature) accumulates across the axial length of a nip or (the width of the web). These differences in expansion coupled with deflection of the rolls make it substantially impossible to obtain acceptable uniform consolidation when a web wider than 40" is consolidated.
Thus in the manufacture of consolidated webs dry formed sheets wherein the web is a significant width i.e. about 40" and wider, problems are encountered in consolidation due to basis weight variation, deflection due to high pressure used in the nip and temperature differential between the rolls.